6. Artificial satellites (e.g. of Earth): Their orbits and uses

You can define a satellite as a smaller object orbiting a larger object and is kept in orbit by the force of gravity.

Natural satellites are those objects that occur naturally in a 'solar system' orbiting a planet.

e.g. our own moon orbiting the Earth, the many moons of Jupiter.

Artificial satellites are those that we, with our technology, put into orbit around our moon, the Earth or our other fellow planets e.g. Mercury, Venus and Jupiter.

When satellites are put into orbit they are given just the right amount of horizontal velocity so that the resultant centripetal force of gravity keeps the satellite accelerating in its a circular orbit.

Reminder: Acceleration and velocity are vector quantities.

The speed of a satellite may be constant, but it is constant changing in direction, so the velocity is constantly changing and change in velocity is acceleration (in this context).

You can vary this horizontal velocity to position satellites at different distances above the Earth's surface.

Most satellites we launch into space are those that orbit our own planet Earth - diagram on right of geocentric orbits - an orbit of anything orbiting the Earth (over 2000 artificial satellites currently orbiting the Earth).

Artificial satellites can now be sent to orbit other planets in our solar system and study them in much greater detail than can be observed by optical telescopes on Earth. You can get much more scientific knowledge including details on the planet's geology, atmosphere and weather systems. You can also orbit satellites around our moon and the moon's of other planets to study them in similar detail.

 

Satellites in polar orbit  (orbit 1 on diagram, actually should be lower than orbit 2)

Satellite orbit 1 passes over (or near) both poles, one after the other in its cycle, and is said to have a polar orbit. It circles around the Earth at about 90^o to the equator.

'Polar satellites' have relatively low orbits and the Earth rotates under them.

The lower orbit means they have to have a faster speed (physics above) than geostationary satellites (below) to stay in orbit which may take as little as a few hours.

Because the orbit time is short and the Earth rotates within the orbit, they can monitor the whole surface of the Earth many times in every 24 hours.

Therefore satellites in polar orbit are used for mapping the landscape, military surveillance (spying!) and producing weather charts for weather forecasting.

Satellites in a geostationary orbit (geosynchronous satellites)  (orbit 2 on diagram)

Satellite 2 travels around in the same direction as the Earth's spin and at the same angular speed are moving in a geostationary (geosynchronous) orbit.

Geostationary satellites have a high orbit that takes ~24 hours to go round the Earth (e.g. at a height of 35786 km above the Earth's surface).

This means they keep a constant position above the Earth's surface because they take exactly one day (23 hours 56 minutes) to complete one orbit.

These geosynchronous satellites are very useful for continuous communications e.g. radio, telephone and TV because you can point transmitters and receivers in their direction and they are stationary relative to each other. Also, the signals can transmitted all around the globe in a fraction of a second ~speed of light.

They are also used for astronomy e.g. the Hubble space telescope producing amazing images of distant star clouds and galaxies - technically it isn't in a geostationary orbit.

On a larger scale, the manned international space station is effectively a giant satellite performing all sorts of scientific functions.